A conventional rotational input device, such as the Surface Dial provided by Microsoft Corporation, includes a dial and a sensing unit for detecting an amount of rotation of the dial. Based on the amount of rotation detected by the sensing unit, the sensing unit transmits a detection signal to a computer according to a human interface device (HID) standard so as to enable a processing unit of the computer to output an operation signal based on the detection signal.
To precisely detect the amount of rotation of the dial, in a case where a physical size of the dial is small, a sensing unit with a high resolution detecting ability is required, resulting in an increased relevant implementation cost. Taking an example that an n-degree (n is an integer) rotation of the dial enables the conventional rotational input device to generate an n-level input for explanation, given that the dial has a diameter of six centimeters, an amount of rotation (i.e., an arc length traveled by a point on a circumference of the dial) equal to 0.52 millimeters
  (            i      .      e      .        ,                  6        ×                  π          360                    =      0.052        )is required to result in a unit level input to the computer which is connected with the rotational input device. When the diameter of the dial is reduced to one centimeter, the amount of rotation required to result in the same unit level input will be reduced correspondingly to 0.087 millimeters
      (                  i        .        e        .            ,                        1          ×                      π            360                          =        0.0087              )    ,which is the amount of rotation that should be detectable by the sensing unit. On the other hand, a larger physical size of the dial alleviates the requirement for a higher-resolution sensing unit, but a resultant heavier weight and cumbersomeness of the dial may adversely affect efficiency of operation of such conventional rotational input device.